Communicating your artistic goals is often not the problem. If you show him or her several examples of the sort of style you mean, along with some commentary on what you like (or don't like) about them, a good artist will usually be able to get the rough idea of what you want, and talk through the details with you.
Usually the problem is communicating the technical goals. These include polygon or vertex limits, texture sizes, the use of texture maps for specular/diffuse/bump/normal/etc (ie. which of these you support), different types of asset budget (eg. for the whole download, for a given level, for any one visible scene, for one model), the way your GUI is skinned, how 2D animations should be packaged (sheets or files), cursor/HUD/sprite sizes, the scale of the models (often 1 unit = 1 metre, but not always), the rotation of the models (which axis is 'forward'?), what the typical lighting will be like, etc.
The more of these you can fix or automate with tooling, the better. But as an indie programmer, your resources will be limited, so instead you will most likely have to resort to clear and unambiguous communication.